linux-kernel - Re: [PATCH v3 1/1] exec: seal system mappings

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Message-ID: <CABi2SkUiJkqyT5=Dun8hHYEFVDcmjP0zz2No-EOm5GFMbLOSBQ@mail.gmail.com>
Date: Tue, 19 Nov 2024 12:57:39 -0800
From: Jeff Xu <jeffxu@...omium.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, jeffxu@...omium.org, akpm@...ux-foundation.org, 
	keescook@...omium.org, jannh@...gle.com, torvalds@...ux-foundation.org, 
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Subject: Re: [PATCH v3 1/1] exec: seal system mappings

Hi Liam,

On Wed, Nov 13, 2024 at 4:54 PM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> > Unlike the aforementioned mappings, the uprobe mapping is not
> > established during program startup. However, its lifetime is the same
> > as the process's lifetime [1]. It is sealed from creation.
>
> Why are you referencing mseal.rst for the uprobe mapping lifetime?  I
> can't find anything in there about uprobe.
>
This should be [2], thanks for checking.

>
> It also can't be used on 32 bit systems, as per your kernel-parameters
> changes (and mseal specification).  This is missing from the changelog.
>
sure, I will add that to the commit msg.

> > +     exec.seal_system_mappings = [KNL]
> > +                     Format: { no | yes }
> > +                     Seal system mappings: vdso, vvar, sigpage, vsyscall,
> > +                     uprobe.
> > +                     This overwrites KCONFIG CONFIG_SEAL_SYSTEM_MAPPINGS
>                              ^^^^^^^^^ overrides ?
sure.


> > -     if (vsyscall_mode == XONLY)
> > -             vm_flags_init(&gate_vma, VM_EXEC);
> > +     if (vsyscall_mode == XONLY) {
> > +             unsigned long vm_flags = VM_EXEC;
> > +
> > +             vm_flags |= seal_system_mappings();
> > +
>
> nit, extra line here.
>
removed.

> But.. this will add the VM_SEALED flag on any 64bit architecture that
> enables the SEAL_SYSTEM_MAPPINGS config.  That will happen by bots with
> random config builds.  I don't know if they have test cases that
> specifically unmap the vmas you are sealing (ppc64 probably tries to
> unmap the vdso).
>
> I do know that I've had syzbot bugs that unmap _all_ vmas.  I'm guessing
> you will get bot notification on these failures for any 64bit
> architecture.  You may want to look into it to avoid such fuzzing
> failures, but we still need this to be tested somehow.
>test_mremap_vdso.c
I found one selftest that could fail:
tools/testing/selftests/x86/test_mremap_vdso.c

I could add tools/testing/selftests/x86/config and add
CONFIG_SYSTEM_MAPPINGS=n there.
as instructed in selftest documentation [1]

[1] https://docs.kernel.org/dev-tools/kselftest.html#contributing-new-tests-details

>
> overwrite or override?  I think the difference is that overwrite implies
> permanence where override doesn't.  I'm fine with either, it just reads
> a bit odd to me.
>
sure, changed to override

> >
> > +config SEAL_SYSTEM_MAPPINGS
> > +     bool "seal system mappings"
> > +     default n
> > +     depends on 64BIT
> > +     depends on !CHECKPOINT_RESTORE
> > +     help
> > +       Seal system mappings such as vdso, vvar, sigpage, vsyscall, uprobes.
> > +       Note: CHECKPOINT_RESTORE might relocate vdso mapping during restore,
> > +       and remap will fail if the mapping is sealed, therefore
> > +       !CHECKPOINT_RESTORE is added as dependency.
>
> You could also add a portion here about your override functionality on
> command line. "this can be disabled or enabled by..."
>
sure.

> I really think having something that can be found by searching for mseal
> is really desirable here.  That is, make menuconfig, press / for search,
> type mseal -> find this feature.  If this was MSEAL_SYSTEM_MAPPINGS,
> searching for seal or mseal would work and would serve to link the
> config option to the mseal document.
>
using "seal" would work here. I will add a note here to mseal.rst for reference.

> Right now there is no information in the help that will allow it to be
> found by 'mseal'.  There is also nothing in the documentation that
> states this exists, which you should probably update with this feature?
>
I will update mseal.rst to include this feature.

Thanks for reviewing.
-Jeff